<!DOCTYPE html PUBLIC "-//W3C//DTD XHTML 1.0 Transitional//EN"
  "http://www.w3.org/TR/xhtml1/DTD/xhtml1-transitional.dtd">


<html xmlns="http://www.w3.org/1999/xhtml">
  <head>
    <meta http-equiv="Content-Type" content="text/html; charset=utf-8" />
    
    <title>fold &#8212; Boost.HigherOrderFunctions 0.6 documentation</title>
    
    <link rel="stylesheet" href="../../../_static/boostbook.css" type="text/css" />
    <link rel="stylesheet" href="../../../_static/pygments.css" type="text/css" />
    
    <script type="text/javascript">
      var DOCUMENTATION_OPTIONS = {
        URL_ROOT:    '../../../',
        VERSION:     '0.6',
        COLLAPSE_INDEX: false,
        FILE_SUFFIX: '.html',
        HAS_SOURCE:  true,
        SOURCELINK_SUFFIX: '.txt'
      };
    </script>
    <script type="text/javascript" src="../../../_static/jquery.js"></script>
    <script type="text/javascript" src="../../../_static/underscore.js"></script>
    <script type="text/javascript" src="../../../_static/doctools.js"></script>
    <link rel="index" title="Index" href="../../../genindex.html" />
    <link rel="search" title="Search" href="../../../search.html" />
    <link rel="next" title="implicit" href="implicit.html" />
    <link rel="prev" title="flow" href="flow.html" /> 
  </head>
  <body role="document">
<table cellpadding="2" width="100%"><tr>
<td valign="top"><img alt="Boost C++ Libraries" width="277" height="86"src="../../../_static/boost.png"></td>
</tr></table>

<div class="spirit-nav">
<a accesskey="p"  href="flow.html"><img src="../../../_static/prev.png" alt="Prev"></a>
<a accesskey="u" href="../../../doc/src/reference.html"><img src="../../../_static/up.png" alt="Up"></a>
<a accesskey="h" href="../../../doc/index.html"><img src="../../../_static/home.png" alt="Home"></a>
<a accesskey="n"  href="implicit.html"><img src="../../../_static/next.png" alt="Next"></a>
</div>
  

    <div class="document">
  <div class="chapter">
      <div class="body" role="main">
        
  <div class="section" id="fold">
<h1>fold<a class="headerlink" href="#fold" title="Permalink to this headline">¶</a></h1>
<div class="section" id="header">
<h2>Header<a class="headerlink" href="#header" title="Permalink to this headline">¶</a></h2>
<div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost/hof/fold.hpp&gt;</span><span class="cp"></span>
</pre></div>
</div>
</div>
<div class="section" id="description">
<h2>Description<a class="headerlink" href="#description" title="Permalink to this headline">¶</a></h2>
<p>The <code class="docutils literal"><span class="pre">fold</span></code> function adaptor uses a binary function to apply a
<a class="reference external" href="https://en.wikipedia.org/wiki/Fold_%28higher-order_function%29">fold</a>
operation to the arguments passed to the function. Additionally, an
optional initial state can be provided, otherwise the first argument is
used as the initial state.</p>
<p>The arguments to the binary function, take first the state and then the
argument.</p>
</div>
<div class="section" id="synopsis">
<h2>Synopsis<a class="headerlink" href="#synopsis" title="Permalink to this headline">¶</a></h2>
<div class="highlight-cpp"><div class="highlight"><pre><span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">F</span><span class="p">,</span> <span class="k">class</span> <span class="nc">State</span><span class="o">&gt;</span>
<span class="k">constexpr</span> <span class="n">fold_adaptor</span><span class="o">&lt;</span><span class="n">F</span><span class="p">,</span> <span class="n">State</span><span class="o">&gt;</span> <span class="n">fold</span><span class="p">(</span><span class="n">F</span> <span class="n">f</span><span class="p">,</span> <span class="n">State</span> <span class="n">s</span><span class="p">);</span>

<span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">F</span><span class="o">&gt;</span>
<span class="k">constexpr</span> <span class="n">fold_adaptor</span><span class="o">&lt;</span><span class="n">F</span><span class="o">&gt;</span> <span class="n">fold</span><span class="p">(</span><span class="n">F</span> <span class="n">f</span><span class="p">);</span>
</pre></div>
</div>
</div>
<div class="section" id="semantics">
<h2>Semantics<a class="headerlink" href="#semantics" title="Permalink to this headline">¶</a></h2>
<div class="highlight-cpp"><div class="highlight"><pre><span class="n">assert</span><span class="p">(</span><span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">z</span><span class="p">)()</span> <span class="o">==</span> <span class="n">z</span><span class="p">);</span>
<span class="n">assert</span><span class="p">(</span><span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">z</span><span class="p">)(</span><span class="n">x</span><span class="p">,</span> <span class="n">xs</span><span class="p">...)</span> <span class="o">==</span> <span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">,</span> <span class="n">f</span><span class="p">(</span><span class="n">z</span><span class="p">,</span> <span class="n">x</span><span class="p">))(</span><span class="n">xs</span><span class="p">...));</span>
<span class="n">assert</span><span class="p">(</span><span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">)(</span><span class="n">x</span><span class="p">)</span> <span class="o">==</span> <span class="n">x</span><span class="p">);</span>
<span class="n">assert</span><span class="p">(</span><span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">)(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">,</span> <span class="n">xs</span><span class="p">...)</span> <span class="o">==</span> <span class="n">fold</span><span class="p">(</span><span class="n">f</span><span class="p">)(</span><span class="n">f</span><span class="p">(</span><span class="n">x</span><span class="p">,</span> <span class="n">y</span><span class="p">),</span> <span class="n">xs</span><span class="p">...));</span>
</pre></div>
</div>
</div>
<div class="section" id="requirements">
<h2>Requirements<a class="headerlink" href="#requirements" title="Permalink to this headline">¶</a></h2>
<p>State must be:</p>
<ul class="simple">
<li>CopyConstructible</li>
</ul>
<p>F must be:</p>
<ul class="simple">
<li><a class="reference internal" href="../../../doc/src/concepts.html#binaryinvocable"><span class="std std-ref">BinaryInvocable</span></a></li>
<li>MoveConstructible</li>
</ul>
</div>
<div class="section" id="example">
<h2>Example<a class="headerlink" href="#example" title="Permalink to this headline">¶</a></h2>
<div class="highlight-cpp"><div class="highlight"><pre><span class="cp">#include</span> <span class="cpf">&lt;boost/hof.hpp&gt;</span><span class="cp"></span>
<span class="cp">#include</span> <span class="cpf">&lt;cassert&gt;</span><span class="cp"></span>

<span class="k">struct</span> <span class="n">max_f</span>
<span class="p">{</span>
    <span class="k">template</span><span class="o">&lt;</span><span class="k">class</span> <span class="nc">T</span><span class="p">,</span> <span class="k">class</span> <span class="nc">U</span><span class="o">&gt;</span>
    <span class="k">constexpr</span> <span class="n">T</span> <span class="k">operator</span><span class="p">()(</span><span class="n">T</span> <span class="n">x</span><span class="p">,</span> <span class="n">U</span> <span class="n">y</span><span class="p">)</span> <span class="k">const</span>
    <span class="p">{</span>
        <span class="k">return</span> <span class="n">x</span> <span class="o">&gt;</span> <span class="n">y</span> <span class="o">?</span> <span class="nl">x</span> <span class="p">:</span> <span class="n">y</span><span class="p">;</span>
    <span class="p">}</span>
<span class="p">};</span>
<span class="kt">int</span> <span class="nf">main</span><span class="p">()</span> <span class="p">{</span>
    <span class="n">assert</span><span class="p">(</span><span class="n">boost</span><span class="o">::</span><span class="n">hof</span><span class="o">::</span><span class="n">fold</span><span class="p">(</span><span class="n">max_f</span><span class="p">())(</span><span class="mi">2</span><span class="p">,</span> <span class="mi">3</span><span class="p">,</span> <span class="mi">4</span><span class="p">,</span> <span class="mi">5</span><span class="p">)</span> <span class="o">==</span> <span class="mi">5</span><span class="p">);</span>
<span class="p">}</span>
</pre></div>
</div>
</div>
<div class="section" id="references">
<h2>References<a class="headerlink" href="#references" title="Permalink to this headline">¶</a></h2>
<ul class="simple">
<li><a class="reference external" href="https://en.wikipedia.org/wiki/Fold_(higher-order_function)">Fold</a></li>
<li><a class="reference internal" href="../../../doc/src/point_free.html#variadic-sum"><span class="std std-ref">Variadic sum</span></a></li>
</ul>
</div>
</div>


      </div>
  </div>
      <div class="clearer"></div>
    </div>
    <div class="footer" role="contentinfo">
    <table xmlns:rev="http://www.cs.rpi.edu/~gregod/boost/tools/doc/revision" width="100%"><tr>
    <td align="left"></td>
    <td align="right"><div class="copyright-footer">
            &#169; Copyright 2016, Paul Fultz II.
        
          Created using <a href="http://sphinx-doc.org/">Sphinx</a> 1.5.6.
          <p>Distributed under the Boost Software License, Version 1.0.
          (See accompanying file <code class="filename">LICENSE_1_0.txt</code> or copy at 
          <a href="http://www.boost.org/LICENSE_1_0.txt" target="_top">http://www.boost.org/LICENSE_1_0.txt</a>)
          </p>
    </div></td>
    </tr></table>
    </div>
  </body>
</html>